The present invention relates to a wavelet transform coding method.
As methods for coding a picture signal with efficiency using an orthonormal transform coding method, subband coding method and wavelet transform coding method are known.
The orthonormal transform coding method realizes an efficient coding by way of the steps of practicing orthonormal transform to an input signal, concentrating power to a specific transform coefficient and utilizing the power bias.
Also, the subband coding method realizes an efficient coding by way of the steps of dividing an input signal into a plurality of frequency bands and utilizing power bias to low frequency band.
The wavelet transform coding method realizes an efficient coding by way of the steps of practicing a o subband class-dividing by recursively dividing a band in low frequency into two, and utilizing concentration of power in low band component.
In these methods, a picture signal is expressed by frequency band and then coded. The signal expressed by frequency bands can be coded with high-efficiency by various methods.
For example, in the orthonormal transform coding method, a method of combination of EOB (End of Block) symbol and a scanning means to transform a two-dimensional transform coefficient to a one-dimensional signal series is known. FIG. 10 illustrates an example of scanning method used for the orthonormal transform. In this method, a coder scans in zigzag two-dimensionally arranged transform coefficients from a low frequency component to a high frequency component, transforms them in a one-dimensional signal series. Then it codes this one-dimensional signal series in order, and generates a symbol called EOB to complete coding in a block when coding of all significant transform coefficients in an arrangement has been completed.
The kind of scanning method in the above examples is only one, however, a method adaptively changing scanning method using difference of local character of a picture signal is also known. For example, this method functions as below. In a picture having many vertical stripes, the low frequency components in vertical direction of power concentrates in horizontal direction as shown in FIG. 11A. On the other hand, in a picture having many horizontal stripes,the low frequency components in horizontal direction of power concentrates in vertical direction as shown in FIG. 11B. Then, corresponding to local character of a picture signal, an efficient coding is realized by selecting and utilizing a scanning pattern as shown in FIG. 11C for a picture having many vertical stripes or a scanning pattern as shown in FIG. 11D for a picture having many horizontal stripes.
For the subband coding method or the wavelet coding method, a method indicated in the Japanese Patent Application No. 3-31502 (1991) is known. In this application, a tree structure expressing the same spatial area is defined and a scanning method scanning from low frequency components to high frequency components in this tree structure is indicated. In FIG. 12, the bands F0 to F9 enclosed by a solid line express an example of frequency dividing in a wavelet transform. Here, the ratio between the sizes of frequency bands corresponds to the ratio of numbers of transform coefficients in the frequency bands. The number of transform coefficients necessary to express the same spatial area is four for F4 to F6, 16 for F7 to F9 when one for F0 to F3. In this example, a frequency band is divided into 10 totally, F0 to F9.
After dividing frequency band in the above manner, a coder starts scanning from transform coefficients belonging to low frequency components to transform coefficients belonging to high frequency components, and codes them as a one-dimensional signal sequence. When coding of all significant transform coefficients in an arrangement has been completed, a signal called EOT (End of Tree) is generated and coding of a tree structure is completed.
In FIG. 12, the number of transform coefficients given by a tree structure expressing the same spatial area is plural in the frequency bands F4 to F9. However, the scanning method indicated in the JP-A-4-245863 (1992) to scan a plurality of transform coefficients included in the same frequency band does not generate continuous zero level signals. In addition, there is or one kind of scanning method; switching adaptively switching over the scanning way corresponding to the picture pattern is not mentioned. Therefore, this method has the disadvantage of coding non-significant transform coefficients that are not necessary to be coded, thus causing deterioration of coding efficiency.